1. Field of the Invention
The present invention generally relates to signal processing systems and, in particular, to an apparatus for detecting a radiation signal and transporting an electric charge that is generated in response to the detected radiation signal to a readout port where the charge can be provided to a signal analyzer or other circuitry.
2. Description of the Related Art
Signal processing systems are employed in a variety of applications. Such systems include communications systems in which a signal transmitter and receiver cooperate to communicate information and sensing systems in which dimensional, positional or other information regarding an object is determined based on analysis of a signal emitted by, reflected by or transmitted through the object. Other signal processing applications include telemetry and telescopy systems, imaging systems, systems for precise measurements of wavelength, velocity and very small distances or thicknesses, rangefinding and guidance systems and a variety of other sensing and communications systems.
All signal information systems include a transmitter that outputs a signal and receiver for receiving the signal output by the transmitter and deriving information from the signal. Generally, the receiver has a detector for detecting the signal and generating charge carriers, i.e., electrons or holes, representative of the signal and an analyzer for processing the information contained in the representative charge. Solid state detectors such as charge-coupled devices (CCDs) have been favored for many applications because of their high quantum efficiency and low noise. The receiver also includes a mechanism for conveying the resulting charge from the detector, radiation sensitive area, to a readout port where it can be provided to an analyzer or other appropriate signal processing circuitry.
Representative of the problems present in many applications that employ receivers and, in particular, receivers that employ CCD technology are the applications in which the location on the detector where particular charges are generated and/or the time when particular charges are generated are important. An example of the former situation is a detector/charge transport mechanism used to resolve a circular radiation pattern produced by an interferometer. In one known type of dector/charge conveyance mechanism for use with an interferometer, a layer of resistive material positioned on a curved detector surface is utilized to convey charge from the detector to the readout port. A DC potential source is connected to terminal portions of the resistive material to produce an electric field which assists in charge conveyance. This configuration of detector and charge conveyance system has many drawbacks. Specifically, critical areas of the radiation pattern, such as the center, cannot be detected. Additionally, current flowing through the resistive layer can result in production of thermally-generated electrons which can interfere with accurate signal detection. Moreover, dark current accumulation, a type of noise that is especially troublesome when the receiver is used to detect low level signals over a long period of time, is a problem on such a receiver. Furthermore, the capacitance of such a configuration can adversely affect the response time of the receiver and thereby limit the application in which the configuration can be used. In addition, due to the materials used in this configuration, fabrication of the detector/charge conveyance mechanism is difficult.
A proposed alternative detector/charge conveyance mechanism for use in an interferometer also poses a number of difficulties. The proposed detector/charge conveyance mechanism includes a detector that is divided into a multiplicity of charge collection sub-areas or pixels, such that each sub-area generates a "packet" of charges representative of the radiation incident on the sub-area. The collected charge packets are periodically read out by clocking the charge packets across the detector surface to a readout port. That is, the charge packets are shifted from one collection sub-area to the next enroute to the readout port by a conventional, clocked shift register. The signal pattern can then be determined based on the sequentially read charge packets. Although such a mechanism theoretically allows for adequate signal resolution, in practice, such a mechanism is relatively complicated to fabricate and often entails complex data processing to derive the desired signal information.
An example of an application in which the time when the detector generates charges is important is a rangefinding system that operates on a radiation transit time principle. In such systems, the range of an object is determined based on the time at which a signal or signals are received by the detector without regard to the location of signal incidence on the detector. In a rangefinder, it is desirable to transfer the charge generated by the detector at a high clocking rate so that the time of signal incidence on the detector and, hence, the range of an object can be accurately determined. For example, the detector may be a typical square or rectangular pixel of a linear CCD array associated with a clocked shift register such that the time of signal incidence is represented by the position of charge within the array at the end of a selected cycle. However, because the maximum clocking rate of such a pixel is inversely related to the size of the pixel, the maximum size of the detector is limited by the desired clocking rate. The size of the detector can, in turn, limit overall signal information system performance.
Based on the foregoing, it is apparent that a new detector/charge transport method and apparatus for use in various signal processing systems is needed. Desirably, such a mechanism should be adaptable for use in location-specific and/or time-specific detection applications and should alleviate or avoid the difficulties discussed above.